There are numerous known medical implants for the treatment of various bone injuries, deformities and disorders. Among these medical implants are spinal fixation systems implanted during a surgical procedure to treat a variety of problems. These treatments include correction of congenital spinal deformities, repair of spinal injuries and fusion of vertebrae to stabilize degenerative conditions and alleviate chronic lower back pain. Several techniques and systems have been developed for correcting and/or stabilizing the spine and/or facilitating spinal fusion. In one type of system, a longitudinal member, such as a bendable rod, is disposed along a portion of the vertebral column, and is fixed to two or more vertebrae along the length of the column by way of a number of fixation elements. Such vertebral fixation elements can include hooks, screws or bolts of various types, each of which are configured to engage specific portions of the vertebral body. In one example, a surgeon can first attach the vertebral fixation elements to the spine in appropriate anatomic positions, and then connect each vertebral fixation element to the spinal rod. Some fixation elements include a head adapted to receive a rod or other longitudinal member. In other systems, a receiving and locking assembly is used to rigidly connect a rod to a fixation element.
U.S. Pat. No. 5,176,680 discloses a device for fixing a spinal rod to vertebral screws. The spinal rod is passed through a split ring which is positioned between the prongs of a vertebral screw having a forked head. The assembly is locked into the desired position by a locking screw threaded between the prongs which and directly onto the split ring.
An assembly for fixing a spinal rod to a bone bolt is shown in U.S. Pat. No. 5,938,663. The spinal rod is passed through a split ring which is disposed within a clamp of a locking assembly. The threaded head of the bolt passes through a pair of openings in the locking assembly and a locking nut is threaded onto the bolt. As the locking nut is tightened, the locking assembly is sandwiched at a particular location between the locking nut and a medial flange or head on the bolt.